Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 042020 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 320 PM EST Sun Dec 4 2016 .SYNOPSIS...
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High pressure will retreat northward tonight. A weak disturbance will affect the area tonight into early Monday, before high pressure briefly builds in again Monday night. On Tuesday, and area of low pressure will split as it moves across the Appalachians to our south. The interior low will weaken Tuesday night, while the coastal low strengthens as it lifts northward overnight. Another low moving across south central Canada may drag a weakening frontal boundary across the area late Tuesday night into early Wednesday. These lows are forecast to weaken through Thursday as another low develops near the Mid Atlantic. As this low lifts northward, it will likely combine with the other two and strengthen to our north Friday into Saturday as high pressure builds to the west, then across the area Saturday night into Sunday.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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High pressure centered just SW of the Delmarva move off the Mid- Atlantic coast this evening. Meanwhile, H5 trough/shortwave combo will move from the Midwest into Great Lakes/OH Valley area. This will allow low pressure over the Gulf Coast to continue to organize and develop as it lifts to the north and east. Dry conditions on tap for this evening. Mid and high clouds will increase and thicken as they spread into the region from the south and west. Overunning precip associated with the developing low should hold off until around midnight or so for SW portions of the CWA. Cold airmass will be slow to depart. Overnight lows will drop into the low to mid 30s for much of the southern portions of the CWA, but for the Lehigh Valley, Pocono Mountains, northern NJ, and the Pine Barrens of NJ, can expect temps to drop into the upper 20s/low 30s. As a result, precip will initially fall in the form of snow from onset through around 6 AM. Temps in the Pine Barrens should warm up relatively quickly into the mid and upper 30s shortly after onset of precip, so would expect a rain/snow mix there. But for the Lehigh Valley, Pocono Mountains, and northern NJ, a prolonged period of snow is expected to fall through daybreak or so, impacting the morning commute with about 1-2" snow. Think totals will fall short of the 3" required for a Winter Weather Advisory, so will not issue one with this forecast package. Instead, a Special Weather Statement will be issued this evening to cover this accumulating snow for the morning commute. One thing the area has going for it in terms of minimizing the QPF tonight is that the models are indicating a split between the northern stream system and the southern stream system. The Gulf low looks to track to the north and east, but late tonight, looks to pass south of the Mid-Atlantic area. Meanwhile, a secondary low will develop associated with the upper trough, and that will lift into western, then central and northern NY state. As a result, 1/4" or so of QPF is likely for the Delmarva, but temps there will be warm enough to keep precip all rain. Up to 1/5" inch liquid QPF possible for northern zones, but if that northern low stays on a track a bit farther to the north, then less QPF, and therefore, less snow would fall. By daybreak, temps will begin to warm up across northern zones to allow some rain to mix with the snow.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
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Rain tapers off fairly quickly from west to east Monday morning, and conditions should be dry throughout the region by lunchtime. Clouds will gradually scour out as high pressure begins to build in from the west. Highs on Monday will top off either right around normal or a couple of degrees above normal, generally in the upper 30s to low 40s across the Poconos, to the upper 40s to around 50 most elsewhere. Highs in the Delmarva will get into the low to mid 50s.
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&& .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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High pressure briefly builds across the area overnight Monday. This high will provide cool and dry weather. This dry weather will only be temporary, as the high pressure will be retreating during the afternoon ahead of an approaching storm system by early Tuesday morning. On Tuesday, an area of low pressure will be moving across the Appalachians to our south, and is expected to split into two separate lows. The interior low is forecast to weaken and likely will be absorbed by a frontal boundary associated with an area of low pressure across south central Canada. This frontal boundary is expected to move across the area Tuesday night into early Wednesday. Meanwhile, the second low to our south will move offshore and strengthen off the coast to our south. As the lows and frontal boundary approach from the south, we expect enhanced moisture and lift to spread across our area as several short waves aloft move across the area. Low clouds and precipitation chances will increase from south to north late in the day into the evening Tuesday, and continue during the overnight. Temperatures will likely be cold enough for some wintry precipitation across portions of the area, especially up north where a period of freezing rain may occur overnight Tuesday into Wednesday morning. By Wednesday morning, the frontal boundary is expected to be north of the area, while the low off the coast weakens east of the area. However, we may still be under the influence of a surface trough associated with this low through the day Wedensday, especially early. As a couple of short wave/vorticity impulses slide across the area, we could continue to have low clouds and precipitation affect the area. Wednesday night could end up being dry as any short waves lift to the north, but clouds will likely linger through the night. By Thursday, a new area of low pressure could form near the Mid Altantic states, although models differ with their placement of where the low develops. This low will lift to the northeast and combine with the two previous lows to our north and strengthen through Friday and Saturday. As the low initially moves across the area Thursday, there is the potential for showers to develop across the area as a strong short waves crosses the area as well. As this low strengthens to our north, a strong west to northwest flow will develop across the area. Steep lapse rates will be in place across the area Friday along with and enhanced low-mid level moisture across the northern portions of the area and farther north. This could lead to lake effect snow or rain showers Friday, which could possibly continue into Saturday up north. High pressure is expected to build across the area Saturday night into Sunday which should provide a dry forecast for the end of the weekend. One thing for sure for the end of the week into the weekend, regardless of precipitation, it will get cold and windy for the latter portion of Thursday, but especially Friday and Saturday.
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&& .AVIATION /20Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions through around 06Z Monday. BKN CIGs of 4000 FT will develop later this evening and going into tonight. NW winds less than 10 KT will become LGT/VRB this evening. MVFR conditions develop in rain for most terminals, but snow at KABE/KRDG. Snow will eventually mix with and change to plain rain by daybreak Monday, and an inch or so of accumulation is possible during that time. By daybreak, IFR CIGs likely as low pressure passes through the region. Could be some spotty IFR VSBYs, but confidence too low to include in TAF at this time. Wind become SE 5 KT or less by daybreak, and behind departing low, winds shift to the W-NW and increase to 5-10 KT. Precip tapers off from west to east between 15Z-18Z, and conditions improve to VFR. OUTLOOK... Monday night...VFR conditions expected. Tuesday-Tuesday night...VFR conditions early, lowering to MVFR/IFR by midday and continues through the night with periods of rain/fog/drizzle likely. Winds likely become gusty 15-20 knots out of the northwest late in the day. Wednesday-Wednesday night...Rain likely ends during the morning, but lower clouds may linger through the day and into the evening. Thursday-Thursday night...Generally VFR, but scattered showers are possible during the day which may temporarily lower conditions. Gusty northwest winds 15-20 knots. Friday..Generally VFR, scattered showers possible, especially across the north. Gusty northwest winds 25-30 knots.
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&& .MARINE...
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Sub-SCA conditions in place tonight through Monday. Winds become LGT/VRB tonight as high pressure departs. A light SE flow will develop by daybreak, but that will shift to the W-NW during the day Monday as low pressure passes through through the waters. Could have gusts to 20 KT on the ocean waters Monday afternoon as the gradient tightens between high pressure building in from the west and the departing low. Reduced VSBYs are expected in rain/fog from after midnight tonight through midday Monday. OUTLOOK... Monday night-Tuesday...Sub-Small Craft Advisory conditions expected. Tuesday night...Small Craft Advisory winds likely. Wednesday-Wednesday night...Winds likely drop below advisory levels, but seas could remain above 5 feet. Thursday-Thursday night...Northwest winds increase to Small Craft Advisory levels late in the day and into the night. Friday...Small Craft Advisory level winds continue, possibly approaching gale force.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson Near Term...MPS Short Term...MPS Long Term...Robertson Aviation...Robertson/MPS Marine...Robertson

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